With TurboTax open:
1. Go to Deductions and Credits (from the menu at the Top)
2. On the first screen "Did you pay any home loans in 2019?" answer yes.
3. On the next screen Who's your mortgage lender? Enter the name of the person who financed your loan.
4. On the next screen choose "The Seller is Financing this Loan and we didn't receive a 1098." or "None of These Apply" (whichever is appropriate)
5. Follow the screen prompts and enter the social security number or EIN of the lender, where requested.
Here is a TurboTax Link with more details regarding mortgage interest. Mortgage Interest entry in TurboTax